This invention relates to pull-outs in tubing and duct systems for conveying gaseous and liquid fluids, and more particularly, to tubing and duct parts made of materials exhibiting superplastic properties and having integral protrusion formations, formed by superplastic forming, by which other matching parts can be attached to produce a fluid-tight system.
Tubing and duct systems for conveying gaseous and liquid fluids are in widespread use in many industries. In the aerospace industry, welded ducts are used in the environmental control system and in the wing de-icing system for conveying heated air from the engine to the leading edges and nacelle inlet nose to prevent ice from forming on those critical surfaces in icing conditions in flight. These and other duct systems have elbows, xe2x80x9cTxe2x80x9d ducts, flanges and other components used to assemble the complete system. A xe2x80x9cT-ductxe2x80x9d is a short length of tubing having an integral tubular protrusion from the duct side wall by which a side duct can be attached, as by welding or coupling hardware, into a duct line. This protrusion is commonly known as a xe2x80x9cpull-outxe2x80x9d.
Two methods for making a tubular part, such as a xe2x80x9cTxe2x80x9d duct, with an integral pull-out are taught in U.S. Pat. No. 5,649,439 issued on Jul. 22, 1997 to David W. Schulz and entitled xe2x80x9cTool for Sealing Superplastic Tubexe2x80x9d. Both methods use gas pressure to superplastically form a portion of a side wall of an end-sealed tube, heated to superplastic temperature in a die, into a side pocket of the die to form the pull-out. The formed tube is cooled and removed from the die, and the end of the pull-out is trimmed off to remove the cap and to give the pull-out a planar lip.
These methods reliably and repeatably produce parts as designed, but have one shortcoming that, in aerospace applications in particular, has significant economic consequences. Since the end cap of the pull-out bulge must remain intact to contain the pressurized forming gas, the material in the cap is not available for use in the pull-out side wall. Accordingly, to prevent excessive thinning of the pull-out, a thicker tube than is required by the engineering specifications for that duct system must be used. That thicker tube, carried just to avoid the excessive thinout of the pull-out lip, can add several pounds to an airplane de-icing duct system, for example. In the aerospace industry, in particular, wherein weight is an important factor in the design of any system, even a few pounds of weight in excess of that required by the engineering specifications is looked upon with disfavor.
Another problem with excessive thinning of the pull-out on a tubular part occurs when the mating duct is welded to the pull-out. Welding of thin-wall ducts and tubing requires careful control of the welding power and speed to obtain a weld bead with the desired penetration and mass, and to avoid burn-through or other over heating problems. Welding a pull-out joint that has been thinned, to a fresh section of straight tubing with a thicker wall, presents a difficult challenge that requires the skills of a master welder. Oftentimes even the best welders are unable to manage keeping an even weld bead or avoid blow-through holes because of the difference in the amount of parent material being melted around the pull-out. Many parts are scrapped because of non-conforming weld bead width, insufficient weld penetration, blow holes, weld-line porosity, inclusions and other defects that can be attributed to the variation of thickness surrounding the pull-out.
The radius area where the pull-out joins the tube is always a high stress area on an airplane de-icing duct system due to bending stresses caused by movement of the wings in flight, thermal stresses and sonic fatigue. All of these factors generate stresses that are transmitted along the spurs of the duct to the joint at the formed pull-out radius where the pull-out meets the mainline section of the straight tube. For this reason, there is a structural benefit in locating the weld bead of the tube welded to the pull-out as far as possible from the pull-out radius, so the stresses that are concentrated at the pull-out radius are not concentrated at the weld bead, since the welding process introduces defects such as porosity, etc. in the weld and decreases the structural load capacity of the duct around the weld.
Another existing tube pull-out production technique is a ball pulling process that is used to produce the same type of aerospace ducting tee""s and joints. A round hole is cut in the sidewall of a tube in a position where the pull-out is to be formed. A ball that is slightly larger in diameter than the hole is pulled through the hole to form a pull-out with the same inside diameter as the outside diameter of the ball. The process is designed in such a way that the ram of a hydraulic actuator can be run up inside the tube through the hole, a ball screwed onto the threaded end of the ram, and the ball pulled through the hole using the hydraulic action of the actuator. The pull-out shape is controlled by a die which has a machine cut draw radius around which the pull-out forms as the ball stretches the material outward.
An enhanced pull-out method has been used wherein the ball is first heated to a temperature of about 1000xc2x0 F. When the pulling process commences, heat from the hot ball is conducted to the tubing material in the region that will be stretched into the pull-out, heating it to an elevated temperature, near the temperature of the ball. A slight increase in ductility is realized by heating the ducting material. For example, the possible elongation of commercially pure titanium made in accordance with Mil Standard Mil-T-9046J, CP-1 at room temperature is about 25%; at 1000xc2x0 F. its possible elongation is about 28%.
The problem with the conventional heated ball pull-out process is cracking and excessive thinout around the lip of the pull-out. The forming stresses and elongations that result during forming are very high and often surpass the formability limits of the material. The strain needed to form the pull-out causes a high scrap rate due to cracking. Aerospace ducting systems are usually designed to approach the minimum thickness to save weight, hence thinout at the lip of the pull-out can reduce the lip thickness below the acceptable minimum. Many parts are scrapped because the pull-out lip is thinner than this engineering designed minimum thickness.
The conventional pull-out forming process has many variables that contribute to the high scrap rate problem. The ductility of alloys used in ducting systems can vary from lot to lot. Elongation differences of only 1 or 2% in the raw material properties can have a significant impact on cracking and thinout.
In addition to variations in the material, it is difficult to precisely locate the hole cut in the tube relative to the position and linear path that the ball travels when the pull-out is made. A misalignment of even 0.005xe2x80x3 can have a significant effect on the elongation of the pull-out sidewalls. Many process failures occur in which the pull-out depth is slightly short on one side and is longer and cracked on the opposite side, resulting from slight misalignment of the hole with the ball travel path.
Because the conventional pull-out forming process causes thinout in the same location that is the most highly stressed, welded duct systems in airplanes have always been designed with thicker tube walls than would otherwise be necessary, thereby increasing the weight of the airplane duct system. The weight is especially undesirable in wing de-icing systems because there is a multiplier effect of weight in the wings.
Thus, there has long been an unsatisfied need in the industry for a process for making pull-outs that does not suffer from excessive thinning of the rim of the pull-out and which avoids cracking or bursting in the highly strained regions around the rim on the pull-out. The benefits of producing a flange, pull-out, or T-duct with reduced thickness variation would extend to both aerospace manufacturing and design capabilities, and also to commercial and industrial applications.
Accordingly, it is an object of this invention to provide an improved method of making a tubular part having a tubular body and a superplastically formed tubular protrusion extending at an obtuse angle from the tubular body and in fluid tight communication therewith. Another object of this invention is to provide an improved reliable method with a low scrap rate of making a tubular pull-out on a duct or other tubular body of superplastic material by which the duct can be connected to adjacent ducts or other tubular members in a fluid conduction system. A further object of this invention is to provide an improved tubular part having an integral pull-out formed by superplastic forming and having an acceptable degree of thin-out at the rim of the pull-out to facilitate connection of ducts or other tubular members to the tubular in an assembly. A still further object of this invention is to provide an improved apparatus for superplastic forming of tubular pull-outs on a tubular part.
These and other objects of the invention are attained in a method of making a superplastically formed integral tubular protrusion in a side wall of tubes for making parts such as tubular elbows and tees, including the steps of inserting the tube in a cavity of a die base and heating the die to a temperature at which the material of which the tube is made exhibits superplastic properties. A distal end of a rod is extended through an opening in the die base and through a hole in the side wall of the tube aligned with the opening in the die. A pull die, having a cross section larger than the hole and about equal to the desired internal cross section of the tubular protrusion, is attached to the distal end of the rod, the pull die is heated to about the superplastic temperature and is pulled through the hole, superplastically forming the tubing material in marginal regions around the hole against surfaces defining the opening in the die base into the tubular protrusion integrally joined to the tube with an integral junction region. Optimal elongations are achieved using optimal strain rates that minimize grain growth and achieve economical production rates. Material thinout around the rim of the pull-out is significantly reduced, and the process enables the use of more extreme pull-out designs. Variations of the process include formed pull-outs on flat or contoured flanges for joining ducting components that are non-circular in cross-section.